Three-piece, wound balls with balata (trans-polyisoprene) covers are typically preferred by professional and low handicap amateur golfers. These balls provide a combination of distance, high spin rate, and control that is not available with an ionomer cover or in one-piece and two-piece balls. However, balata cuts easily, and lacks the durability required by the average golfer.
Two-piece golf balls, which are typically used by the average amateur golfer, provide a combination of durability and maximum distance that is not available with balata covered balls. These balls comprise a core, formed of a solid sphere which typically comprises a polybutadiene based compound, encased in an ionomer cover, such as SURLYN.RTM.. Golf ball cover ionomers are, typically, copolymers of an olefin and an unsaturated carboxylic acid in which at least a portion of the carboxylic acid groups have been neutralized with a metal ion. These balls are extremely durable, have good shear resistance and are almost impossible to cut. However, the durability results from the hardness of the ionomer, which gives such balls a very hard "feel" when struck with a golf club that many golfers find unacceptable.
Golf ball manufacturers have attempted to produce golf ball covers that provide the spin rate of balata with the cut resistance of an ionomer by forming blends of high hardness and low hardness ionomers, e.g., U.S. Pat. Nos. 4,884,814, 5,120,791, 5,324,783, 5,415,937 and 5,492,972. However, none of the disclosed ionomer blends have resulted in the ideal balance of carrying distance, coefficient of restitution, spin rate and initial velocity that would approach the highly-desirable playability of a balata covered golf ball.
Manufacturers have also attempted to form blends consisting essentially of hard ionomers with softer, nonionomer polymers to soften the golf ball and improve its feel and spin rate. However, this approach has proven to be difficult because the ionic character of ionomers imparts a highly polar nature to these materials. Therefore, ionomers and other non-ionic polymers, such as balata, and polyolefin homopolymers, copolymers, or terpolymers that do not contain ionic, acidic, basic, or other polar pendant groups, have not been successfully blended for use in golf ball covers. These mixtures often have poor mechanical properties such as inferior tensile strength, impact strength, and the like. Hence, the golf balls produced from these incompatible mixtures will have inferior golf ball properties such as poor durability and cut resistance on impact.
In light of the inferior properties imparted to golf balls by the ionomer and nonionomer polymer blends of the prior art, as described above, other approaches to forming such blends have been attempted by manufacturers. For example, U.S. Pat. Nos. 4,986,545, 5,098,105 and 5,359,000 all disclose compatible or miscible blends between ionomers and nonpolar polymers which have been modified by the addition of polar functionality. Compatibility is accomplished by imparting polar functionality to the nonionomer polymer through a reaction with maleic anhydride. Alternatively, a compatibilizer component has been added to provide or enhance the compatible nature of such blends; see, for example, U.S. Pat. Nos. 5,155,157 and 5,321,089 and Japanese patent application 6192512 A (1994). The compatibilizer material is often a block copolymer, where each block has an affinity for only one of the blend components to be compatibilized, or an epoxy containing compound. However, in each of these disclosures or publications, a costly chemical modification step or an added compatibilizer component is required to compatibilize a blend of one or more ionomers with a polymer that is otherwise incompatible with the ionomer. None of the above disclosures or publications teaches a compatible or miscible blend of an ionomer with a polyamide-containing polymer in the absence of a compatibilizing agent.
It is known in the polymer art that polyethylene-based ionomers may act as emulsifiers or compatibilizers when added to otherwise incompatible blends of a polyamide with a polyolefin. J. M. Willis et al., J. Materials Sci., 26:4742 (1991). For example, uncompatibilized polyamide/polyethylene blends are known to be immiscible or incompatible, that is, characterized by two distinct phases. However, they can be compatiblized by adding a polyethylene-based ionomer during melt blending. Blends of thermoplastics, including nylon, with ionomers having improved low temperature properties, such as flex-crack resistance, are the subject of U.S. Pat. No. 4,801,649 and European Patent Application A2 148,632. Furthermore, blends of nylon 6,6 and ionomer resins are known for their high toughness. (See: O. Olabisi, Encyclopedia of Chemical Technology, 3rd Edition, 18: 474 (1982)). However, the use of polyamide-ionomer blends in golf ball compositions or in golf balls is not disclosed by these references.
U.S. Pat. No. 5,244,969 to Yamada discloses golf ball covers comprising a blend of two specific ethylene-acrylic acid copolymer ionomer resins and less than 20% by weight of a polyamide. Yamada does not claim or even disclose, however, a blend of a single ionomer resin and a polyamide.
U.S. Pat. No. 5,427,377 to Maruoka discloses reclaimed golf balls made by applying a thin skin of an ionomer resin optionally blended with a polyamide to resurface a golf ball which has been previously ground so that the dimples and damaged areas are removed. However, the reclaimed golf balls have inferior flying distance and durability compared to virgin golf balls.
Several patents disclose blends of polyamide elastomers and ionomers. For example, U.S. Pat. No. 4,858,924 to Saito et al. discloses the use of 3-35 wt % of a flexible resin, which can include a polyamide elastomer, blended with an ionomer for use as the cover of a golf ball. The cover blend is required to have a flexural modulus within the range of 21,000-64,000 psi (1500-4500 kg/cm.sup.2). The polyamide elastomer is said to be incompatible with the ionomer such that it forms a discrete phase dispersed in the ionomer. Saito et al. '924 do not disclose any compatible polyamide-containing polymer and ionomer blends or any hardness requirements for any of the components or for the overall blend.
U.S. Pat. No. 4,919,434 to Saito discloses the use of a polyamide elastomer blended with an ionomer for use as the inner or outer cover of a golf ball. Described only as elastomeric, the polyamide elastomer material is said to have a low flexural modulus and hardness. However, the exact chemical composition or structure of the polyamide elastomer is not disclosed. When blended with an ionomer, the polyamide elastomer-ionomer blend outer cover layer is said to have a flexural modulus of 28,000-71,000 psi (2000-5000 kg/cm.sup.2). However, Saito '434 is silent on the hardness characteristics of the blends and of their components.
U.S. Pat. No. 5,556,098 to Higuchi et al. discloses the use of a three-layer golf ball with a soft middle layer composed of a blend of a polyamide elastomer and an ionomer, such that the JIS C hardness of the blend is less than 80. The exact chemical composition or structure of the polyamide elastomer is not disclosed other than that it is said to be a thermoplastic elastomer. Higuchi et al. are silent on the flexural modulus characteristics of these blends and of their components.
It should be noted that none of the above publications on polyamide elastomers and their ionomer blends disclose blends of polyamide/ionomer or copolyamide/ionomer.
Block copoly(amide-ether)s, available commercially as PEBAX.RTM. from Elf Atochem S.A., have been disclosed as cover compositions for golf balls, for example, by Deleens et al. in U.S. Pat. No. 4,234,184. These materials are formed by reacting a polyamide that has been end-capped to form an .alpha.,.omega.-dicarboxylic copolyamide with a dihydroxy-terminated polyether. Deleens et al. also provide for the possibility of blends containing the block copoly(amide-ether), but only with a minor proportion of a compatible polymer with a melting point and hardness comparable to the block copolymer.
Cores for multi-piece golf balls, as well as one-piece balls, comprised of blends of an ionomer and a block copoly(amide-ether) are disclosed in U.K. Patent Application GB 2,164,342 A. However, this reference is silent on the hardness and flexural modulus characteristics of the blends or their components.
U.S. Pat. No. 5,253,871 to Viollaz discloses the use of at least 10% of a block copoly(amide-ether), optionally blended with an ionomer, for use as the middle layer of a three-layer golf ball. The Shore D hardness of the block copolymer is said to be within the range of 30-40 Shore D hardness while the corresponding hardness of the ionomer component is said to be between 55-65 Shore D. The overall hardness of the middle layer is said to range from 20-50 Shore D. The cover may also be a block copoly(amide-ether) and ionomer blend but its overall hardness must be greater than that of the adjacent middle layer. However, Viollaz is silent on the flexural modulus characteristics of the blends or their components.
It should be noted that none of the above publications on block copoly(amide-ether)s and their ionomer blends disclose blends of polyamide/ionomer or copolyamide/ionomer.
UK patent application GB 2299999 A discloses a three layer golf ball with a soft cover layer and a hard intermediate layer. The intermediate layer is said to contain at least 5% by weight of a high-rigidity polyamide resin, optionally blended with an ionomer. Preferred polyamides are said to have a stiffness or bending modulus of 140 to 252 kpsi. The bending modulus of the intermediate layer composition is said to range from 56,000 to 112,000 psi, preferably 70,000 to 112,000 psi, while its Shore D hardness is said to range from 65 to 90. The cover layer of the three layer golf ball is said to comprise ionomer resin with a stiffness of 14,000 to 35,000 psi and Shore D hardness of 56 to 64.
It should be noted that this publication does not disclose a blend of copolyamide/ionomer, nor does it disclose a blend of polyamide/ionomer present in a cover layer, nor does it disclose a blend of polyamide/ionomer present in an intermediate layer which is softer than the cover layer.
None of the blended compositions described above provides the combination of durability and distance that is provided by two-piece golf balls with ionomer covers and the high spin rate and control that is available with three-piece, wound golf balls with balata covers. Therefore, there remains a need for a method of making golf ball cores, intermediate layers and covers that provides for the blending of thermoplastic ionomers with other polymers without the need for compatibilizers which complicate and raise the cost of such a process, to provide one-piece, two-piece and/or multi-layer golf balls with the durability and distance of a SURLYN.RTM. covered two-piece ball and the feel, click, and control of a balata covered three-piece ball. The present invention provides such a method, and produces such a product.